1. Field of the Invention
The present invention relates to an apparatus for measuring the thickness of a thin film.
2. Description of the Prior Art
It is well known that interference fringes can be observed by projecting a light beam onto a thin film having parallel top and bottom surfaces which is optically transparent. The thickness of a thin film can be measured utilizing this interference phenomenon.
Measuring methods for practical use are classified into two methods as follows:
(1) Method utilizing an interferometer
An interferogram is obtained by moving a movable mirror in a system using a Michelson type interferometer with a white light. The thickness of a thin film can be estimated by measuring the separation of the central and side bursts of the obtained interferogram. This interferogram can be also obtained using an FT-IR (Fourier Transformation-Infrared) spectrometer for general use.
(2) Method utilizing a wavelength monochrometer
In a system therefor, a source of white light and a wavelength disperser such as a diffraction grating or a prism are used. The wavelength of an incident light is mechanically varied and an interference spectrum is obtained. This method is essentially the same as that employed in a spectrophotometer of the dispersion type. Alternatively, a spacial interference spectrum generated by a polychrometer is measured by a linear array detector without scanning a monochrometer mechanically.
However, both of these methods still have some problems to be solved in the view point of practical use in the case that they are applied to a production line for a thin film wherein a reasonable resistance against oscillations and the environment are required.
In the former method (1), the initial adjustment and pricise control of a translational movement of a moving mirror are difficult and, furthermore, this system is unstable due to mechanical oscillations and the side bursts of an interferogram cannot usually be clearly observed.
In the latter method (2), it is difficult to obtain a reasonable mechanical stability and a high-speed thickness measuring of a running film since the monochrometer has to be scanned mechanically. In the method wherein the linear array detector is used, an applicable range of wavelength is limited, and a high sensitivity cannot be obtained and, also, the magnification of the optical system is required to be constant though a fast processing time can be obtained.
A key point of the method for accurately measuring the thickness of a thin film utilizing the optical interference phenomenon is to obtain a clearly distinguishable and stable interference fringe pattern.
The visibility of the interference pattern depends mainly on the transparency of a film to be measured. Due to this reason, films measurable by the conventional method are limited and restricted. In other words, the method cannot be applied to a film having a low transparency such as a translucent film, a coating layer on a metal plate or the like.
In order to extend the applicability to various films, the choice of a light source is important. A laser is ideal as a light source because of its monochromatic and coherent characteristics, directional property and high intensity. But, the laser is not applicable to the method mentioned above since it is a monochromatic light.
On the contrary, in U.S. Pat. No. 4,660,980, there is disclosed an apparatus for measuring the thickness of a thin film wherein a coherent light beam is used to scan the thin film and the difference between an order of interference fringes obtained for one scan by the coherent light beam is counted to detect the thickness of the thin film.
However, an optical system used in the apparatus is not easy to adjust since a scanning optical system and a detecting optical system are formed separately. Furthermore, it is difficult to calibrate an incident angle of the incident beam accurately.